Use a backup SPG with air integration?

[dmaziuk]

I could totally be wrong, but from anecdotes I've read I have the impression the more likely reason for a mechanical SPG to read falsely high would be contamination inside the gauge mechanism. Not a failure of the Bourdon tube itself. More like a salt crystal or piece of grit happens to get inside and causes it to mechanically bind. That kind of failure or any type that is analogous to that seems pretty unlikely to happen to a pressure transducer.

Well, yeah, but by the same token salt water getting into WAI sensor casing could short something, perhaps the transistor responsible for calibration adjustment... It is more likely that it'll fry the whole thing though, no false readings then. I would think IRL the most likely cause for salt/water inside is mechanical damage, WAI transmitters might be more succeptible to that thanks to lifting tanks by that convenient plastic handle.

 

[scubadada]

This topic has been discussed many, many times on SB and the comments in this thread are similar/identical to previous debates. As usual, it is to each their own, a personal decision.

I have been diving a hoseless AI, Oceanic VT3, for 7 1/2 years, nearly 1000 dives, more than 1000 hours. I have always dived with with a backup computer and SPG. In all that time, I have used backup exactly twice. On one occasion, I did not check the battery on my primary computer after it had sat dormant for a month or so. The computer went dead during the 1st dive and I used my backup computer and SPG to finish the dive. After replacing the battery, I was back in business. I pay more attention to my battery status than I used to and replace it promptly, as soon as I get a low battery signal. More recently, my transmitter went out during a dive and I finished the dive using my SPG. I replaced the transmitter with another from my son's VT4 and finished a week of dives. Oceanic replaced the transmitter with a brand new one, as part of their full service and calibration program, for just $120. If I had not had a backup SPG, I would have missed at least a dive, more if I didn't have an SPG in my save a dive kit or needed to purchase one.

I love to dive and loathe missing a dive or part of a series due to equipment malfunction. For me, small price to pay to carry a backup computer and SPG. I go into each dive knowing I could easily deal with a problem, for that dive or for a dive trip. No right or wrong, as I started out, to each their own, a personal decision.

 

[Dan]

SPG doesn't require battery that could run out of juice & lose signal during the dive. Also, much less maintenance time & cost (require no battery replacement).

 

[stuartv]

What less failure points are you referring to?

AI transmitter:

- One static O-ring in the HP port. If it fails, you lose gas.
- One static O-ring in the battery compartment cover. If it fails, you do not lose gas.

SPG:

- One static O-ring in the HP port. If it fails, you lose gas.
- 2 dynamic O-rings on the HP spool (the part inside the connection between the HP hose and SPG). If either fails (I believe) you lose gas.
- One static O-ring (?) sealing the SPG itself. If it fails, you lose gas.
- HP hose. A 26" (or however long you choose to use) hose that, if it ruptures, you lose gas.

SPG doesn't require battery that could run out of juice & lose signal during the dive. Also, much less maintenance time & cost (require no battery replacement).

My original transmitter is almost 3 years old (~180 dives) and still on its original battery. If you have the battery changed annually, when you do you annual reg maintenance (you do have your reg maintenance done, don't you?), it doesn't seem like really any extra maintenance. Just a few extra bucks when you get the maintenance done. I definitely don't see how you can support the statement "much less maintenance time and cost." I've let mine go so long because it's my first one and I just want to see how long it will actually go on one battery. Once it finally shows as "Low" on the computer's display (for Tmtr Batt status), I will replace it and then stick to something like once every 2 years or 150 dives for replacement. And, you know, you probably will need to replace your SPG's HP hose every now and then, too. Probably not every 2 years, but I still don't see any significant difference in maintenance. Scuba gear needs maintenance. Replacing a battery every year or 2 just doesn't even create a blip on my radar.

 

[stuartv]

Well, yeah, but by the same token salt water getting into WAI sensor casing could short something, perhaps the transistor responsible for calibration adjustment... It is more likely that it'll fry the whole thing though, no false readings then. I would think IRL the most likely cause for salt/water inside is mechanical damage, WAI transmitters might be more succeptible to that thanks to lifting tanks by that convenient plastic handle.

I generally agree. Except the part about lifting a tank using WAI as a handle. I read about it as a concern here on SB. I've never seen it happen IRL. But, I have had my SPG (when I was using one) swing loose and whack stuff. Or be hanging down onto a bench where someone could drop a tank on it. I have had one blow an O-ring on the HP spool and start dumping air. The same one did not have the HP hose tightened onto the SPG tight enough and, over time, gradually just came unscrewed from the hose. It's supposed to swivel, so failing to notice that, when I turned it to look at it, it was unscrewing rather than swiveling doesn't seem like a completely gross oversight on my part. In contrast, there are no moving parts in the AI transmitter. No moving parts means no dynamic O-rings, means less likely to blow.

I don't know how an SPG gets a salt crystal in it, but I have read posts here on SB from people who said their SPG started sticking and that that was a reason. As you say, if something like that got in an AI transmitter, it would be more likely to fry than to display believable-but-wrong readings. I think the SPG sticking is a far bigger safety issue.

 

[Dan]

AI transmitter:

- One static O-ring in the HP port. If it fails, you lose gas.
- One static O-ring in the battery compartment cover. If it fails, you do not lose gas.

SPG:

- One static O-ring in the HP port. If it fails, you lose gas.
- 2 dynamic O-rings on the HP spool (the part inside the connection between the HP hose and SPG). If either fails (I believe) you lose gas.
- One static O-ring (?) sealing the SPG itself. If it fails, you lose gas.
- HP hose. A 26" (or however long you choose to use) hose that, if it ruptures, you lose gas.



My original transmitter is almost 3 years old (~180 dives) and still on its original battery. If you have the battery changed annually, when you do you annual reg maintenance (you do have your reg maintenance done, don't you?), it doesn't seem like really any extra maintenance. Just a few extra bucks when you get the maintenance done. I definitely don't see how you can support the statement "much less maintenance time and cost." I've let mine go so long because it's my first one and I just want to see how long it will actually go on one battery. Once it finally shows as "Low" on the computer's display (for Tmtr Batt status), I will replace it and then stick to something like once every 2 years or 150 dives for replacement. And, you know, you probably will need to replace your SPG's HP hose every now and then, too. Probably not every 2 years, but I still don't see any significant difference in maintenance. Scuba gear needs maintenance. Replacing a battery every year or 2 just doesn't even create a blip on my radar.

I haven't replaced & do any maintenance on my SPG for 4 years so far. It's my backup gear so it's not high on my list of concern. However, I do take good care of it to make it maintenance free to as long as it can get by:
1. Make sure no moisture getting into the 1st stage regulator during air tank change out, blow dry the salt water out of the regulator valve seat before changing the air tank.
2. I don't let the boat crew to mess with my gear & I install the reg set up myself.
3. Hang / store my reg set in dry storage, keeping the hoses from pinching or bending too tight in the bag while traveling.
4. Not playing with the SPG swivel to keep off the dynamic orings from wearing off and carry a spare swivel kit with o-rings.
5. I routinely check the SPG reading with that of from the pressure transducer of my Suunto Cobra3 from full (~3000psig) to empty tank (~500psig).

I haven't experienced any o-ring problem in my 12 years of diving other than the o-ring on the rented air tank connection to my regulator yoke. I have a habit of checking the leak by either listening to any hissing sound coming out of the connection during pressure testing the system after air tank change out or spit on the connection & see for bubbles forming if the area is too noisy to listen to any faint hissing of leak. So, I pretty much ruled out the problem due to o-ring failure from my gear.

We'll see how long this SPG set up last without any maintenance.

 

[dmaziuk]

Except the part about lifting a tank using WAI as a handle. I read about it as a concern here on SB. I've never seen it happen IRL.

I find it very believable, though, because if you lift a tank by the 1st stage with one hand, you're almost guaranteed to press on the transmitter housing sideways. Banging the dangling SPG or dropping the tank of top of it is very believable too.

I have a habit of checking the leak by either listening to any hissing sound coming out of the connection during pressure testing the system ...

Courtesy of a DFB divemaster on Bonaire: if the o-ring blows at that point, your eardrum will fly out the other ear.

 

[Dan]

....Courtesy of a DFB divemaster on Bonaire: if the o-ring blows at that point, your eardrum will fly out the other ear.

the DFB divemaster has witnessed it happened I ASS-U-ME.

Thanks for the tip. From my experience, a blown o-ring usually happens when a rookie tries to forcely loosen the yoke while the connection is under pressure. The rookie with brute strength loosen the yoke without depressuring the connection by closing the tank valve, puffing the 2nd stage and check the SPG for any pressure. I'm guilty of that

If the yoke is tightly screwed onto the tank valve connection and pressed against the o-ring, the o-ring won't blow, since it has no room to blow out to and it is under compression by the yoke O-ring seal face. Bad o-ring would just leak air a bit in a slight hissing sound.

Obviously I won't be rushing to hear the leak sound as soon as I open the valve. First, I'll look into the SPG to see how much pressure I have in the tank. Check any movement (drift) of the needle and puff the 2nd stage & Octopus a couple times to see if their seat are still good. Then listen to the yoke connection for leaky hissing sound.

 

[dmaziuk]

I cup my hand over the yoke. If it leaks so much I can still hear it, then it's a problem, otherwise it's good enough for me.

 

[stuartv]

From my experience, a blown o-ring usually happens when a rookie tries to forcely loosen the yoke while the connection is under pressure. The rookie with brute strength loosen the yoke without depressuring the connection by closing the tank valve, puffing the 2nd stage and check the SPG for any pressure.

An O-ring on the spool in my 2 month old SPG assembly blew after I turned my gas on and had walked away, heading for my truck (10 feet away).